Reaction Mechanism of Thiourea as Suppressor in Electroless Cu Deposition Process

For more precise control of the electroless deposition process, reaction mechanisms of additives should be elucidated well. Particularly, in the case of the electroless deposition of Cu, behavior of thiourea (TU), which is one of the most widely used suppressor, molecular level understanding is significant. Although analyses of TU have been reported by numbers of researchers [1,2], its suppression mechanism has not been systematically understood. In this study we attempted to elucidate suppression mechanism of TU in electroless Cu deposition with formaldehyde as a reducing agent.

Since the electroless Cu deposition reaction involves both oxidation of formaldehyde and reduction of Cu²⁺ ion, the effect of TU on each reaction should be evaluated separately. For this, we employed “separated bath” system [3]; the bath without Cu²⁺ ion (oxidation bath), in which the only oxidation of formaldehyde proceeds, and the bath without formaldehyde (reduction bath), in which the only reduction of Cu²⁺proceeds, are connected with copper wire as shown in Fig. 1. The conditions of each bath are listed in Table 1.

When TU is added only to the oxidation bath, current density steeply decreased as shown in Fig. 2 (a), indicating that TU inhibits the oxidation of formaldehyde. On the other hand, when TU is added only to the reduction bath, current density slightly increased as shown in Fig. 2 (b), indicating that TU accelerates the reduction of Cu²⁺, probably due to the capturing effect of TU on Cu²⁺around the interface [4]. Overall, TU suppresses the Cu deposition reaction by inhibiting the oxidation reaction of reducing agent.

To elucidate the inhibition effect of TU on the reaction of formaldehyde, the adsorption states of TU and formaldehyde are calculated by DFT. Comparing these results, it is indicated that the adsorption energy of TU is much higher than that of formaldehyde. This shows that the adsorption of TU is much favorable on Cu substrate rather than that of formaldehyde. Hence, TU should inhibit the adsorption of formaldehyde by occupying the adsorption sites. Furthermore, from the DFT analysis, combined with SERS measurement, monomeric TU was indicated to adsorb on Cu surface via its NH₂group because of the solvent effect.

Acknowledgements

This work was financially supported in part by Grant-in-Aid for Scientific Research (A) and “Development of Systems and Technology for Advanced Measurement and Analysis” program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

References

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